You will need a device (transducer) that is able to convert the blood pressure reading into a proportional electrical signal.

This signal will then have to be amplified to a reasonable level that it can be analysed. Filters will need to be administered to eliminate any noise or hum brought on by power supplies or other random noise sources.

Once the signal is amplified and filtered to a reasonable level it will need to be digitized in order for the XBee device to transmit it as data. To do this, you will need to use a device with analog-to-digital conversion (ADC) capabilities to convert the amplified/filtered analog reading into 1s and 0s that can be received and understood by a computer. These ADCs need to have input analog voltages between 0 and 5V usually so the input signal cannot contain negative voltages, so before passing the signal to the ADC, it will need to be shifted upwards in voltage to make sure it is entirely positive.

Once you have the voltage amplified/filtered/shifted, you can pass it to an ADC to convert samples into binary numbers. You will then need a device that can communicate with an XBee under an identified protocol (usually USB or UART) in order to get your data out.

Luckily, a microcontroller has ADC and UART on-board and sometimes USB.